Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Retained Earnings
Beginning Balance (shares) at Apr. 30, 2015		38,886,165
Beginning Balance at Apr. 30, 2015	$ 875,229	$ 56,274	$ 818,955
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	225,982		225,982
Dividends declared	(34,342)		(34,342)
Exercise of stock options (shares)		108,100
Exercise of stock options	3,717	$ 3,717
Issuance of common stock (shares)		32,717
Issuance of common stock	2,762	$ 2,762
Tax benefits related to nonqualified stock options	2,702	$ 2,702
Stock-based compensation (shares)		28,588
Stock-based compensation	7,413	$ 7,413
Ending Balance (shares) at Apr. 30, 2016		39,055,570
Ending Balance at Apr. 30, 2016	1,083,463	$ 72,868	1,010,595
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	177,485		177,485
Dividends declared	(37,534)		(37,534)
Exercise of stock options (shares)		69,150
Exercise of stock options	2,357	$ 2,357
Issuance of common stock (shares)		28,138
Issuance of common stock	3,526	$ 3,526
Repurchase of common stock (shares)		(443,800)
Repurchase of common stock	(49,374)	$ (49,374)
Stock-based compensation (shares)		56,763
Stock-based compensation	$ 10,697	$ 10,697
Ending Balance (shares) at Apr. 30, 2017	38,765,821	38,765,821
Ending Balance at Apr. 30, 2017	$ 1,190,620	$ 40,074	1,150,546
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	317,903		317,903
Dividends declared	(39,060)		(39,060)
Exercise of stock options (shares)		40,377
Exercise of stock options	1,377	$ 1,377
Repurchase of common stock (shares)		(1,997,800)
Repurchase of common stock	(215,434)	$ (57,186)	(158,248)
Stock-based compensation (shares)		65,924
Stock-based compensation	$ 15,735	$ 15,735
Ending Balance (shares) at Apr. 30, 2018	36,874,322	36,874,322
Ending Balance at Apr. 30, 2018	$ 1,271,141	$ 0	$ 1,271,141